i like the deer fending here nice and tall should keep horse in if the fields run along the motorway .http://www.dft.gov.uk/ha/standards/mchw/vol3/section1/h_series.pdf
I agree about the fly grazing but am not so sure about the criminal offence or manslaughter.
Trouble is that if you write a law to deal with a specific problem, such as traveller horses fly grazing and escaping onto road, you end up turning innocent people into criminals.
I always remember talking to a girl who kept horses near a motorway. Someone cut the chains and padlocks on TWO gates to use her fields as access for a burglary of a third parties premises. Her horses strayed onto the road and caused an accident. She was being sued for the injuries. Now you can see why, the people actually responsible were not caught. But if it was a criminal offence she would have been charged. Because the bottom line is that her horses did stray onto the road.